1. Field of the Invention
This invention relates to an image signal recording and reproducing system for recording an image signal on a recording medium and reproducing the recorded image signal from the recording medium.
2. Description of the Related Art
An electronic still video camera (hereinafter referred to as a floppy camera) has been commmercialized. The floppy camera is arranged to record, as a still image signal, an image signal obtained from an image sensor such as a CCD (charge-coupled device) or the like on a disc-shaped magnetic recording medium or a video floppy disc. Unlike a silver-halide type photograph, the image recorded by the floppy camera can be instantly reproduced and appreciated either on a TV monitor set or in the form of a hard copy without any developing process.
However, the picture quality, or particularly the resolution, of the image obtained by the floppy camera is determined almost solely by the number of picture elements of the image sensor. More specifically, the resolution in the horizontal direction is limited by the number of picture elements in the horizontal direction. The resolution in the vertical direction is determined by the number of scanning lines obtained according to the current NTSC system.
To solve this problem, a method has been considered for obtaining an image with a degree of resolution of about 1300.times.1000 picture elements which is equal to the HDTV (high definition TV) while retaining interchangeability with the conventional still video system. FIG. 1 of the accompanying drawings shows this method. Referring to FIG. 1, the 1300.times.1000 picture elements include picture elements indicated by a mark ".largecircle." and picture elements indicated by a mark "X". There are about 650 picture elements ".largecircle." or "X" in the horizontal direction and 500 picture elements ".largecircle." or "X" in the vertical direction. The picture element signals of all lines consisting of these picture elements ".largecircle." or "X" are recorded on the video floppy disc by assigning them to four tracks A, B, C and D. In reproducing the record, the picture element signals separately recorded in these tracks on the video floppy disc are recombined on a semiconductor memory and interpolation is made between the picture elements, so that an image consisting of 1300 (horizontal).times.1000 (vertical) picture elements can be obtained by means of a printer or the like as a print which withstands comparison with a silver-halide photograph.
In this case, the image signal is output from the image sensor in the following manner: electric charges accumulated at the image sensor must be read out within a shortest possible period of time for the purpose of preventing S/N ratio deterioration due to an increase in a dark current. To meet this requirement, the electric charge of each of the lines A and B is read out, for example, within a period during which a magnetic head is tracing one track on the video floppy disc and the electric charge on that of each of the lines B and C while the head is tracing a next track on the video floppy disc.
More specifically, the operation is as follows: the magnetic head has two channels 1 and 2 as shown in FIG. 2. A signal obtained from the line A on the image sensor is supplied to the channel 1 simultaneously with a signal which is obtained from the line B and supplied to the channel 2 of the head while the video floppy disc makes one turn. Then, while the disc makes another turn, the magnetic head is shifted to distant of a two-track-pitch to supply signals obtained from other lines C and D on the image sensor simultaneously to the channels 1 and 2 of the two-channel magnetic head.
In this instance, the position in a track pattern of the horizontal synchronizing signal (hereinafter referred to as the H-sync signal) of the image signal recorded in each track is preferably in a so-called H alignment (H: a horizontal synchronizing period) between the tracks A and B and between the tracks C and D as well known and as shown in FIG. 2 in consideration of a cross-talk which tends to take place in recording between the channels of the magnetic head. With the floppy camera arranged in this manner, in reproducing a frame image with a two-channel magnetic head of the conventional still video reproducing apparatus, the frame image can be simply reproduced by reproducing the record from the tracks B and C, because the H alignment of the tracks A and B deviates 1/2 H from that of the tracks C and D.
In the method shown in FIG. 1, the picture elements forming an image signal are arranged by shipping every other picture element in recording the data of the points ".largecircle." and "X" on a video floppy disc according to the currently practiced still video format. In accordance with this method, "sampled values" transfer is performed instead of "wave form" transfer. This necessitates that the transfer function of the total transfer path required for the magnetic recording or reproducing and frequency modulation or demodulation is in conformity with the well-known first criterion of Nyquist.
Further, in reproducing, sampling points used in recording must be accurately re-sampled. It is, therefore, necessary to use a time base corrector (TBC) for accurately correcting, within the H period, any time base variations, occurring in the magnetic recording and reproducing systems.
For this purpose, it is conceivable to record a continuous pilot signal (fT) by frequency-multiplexing it with the image signal between the frequency band of a frequency-modulated color-difference line-sequential signal (C) and that of a frequency-modulated luminance signal (Y). In accordance with this method, the frequency of the pilot signal fT is set, for example, between 2.5 and 3.5 MHz. Therefore, the time base correction can be accomplished at a high degree of precision.
The above-stated pilot signal is not included in the currently practiced still video format. If the pilot signal fT is recorded by frequency-multiplexing it with the image signal interchangeability would be lost. This is a serious problem.
In a conceivable solution of this problem, an ID signal, which has conventionally been recorded on a video floppy disc along with the image signal by DPSK (differential phase shift keying)-modulating a given carrier signal according to information on the year, month, day, hour, minute, second, etc. corresponding to the image signal, may be utilized as the pilot signal by recording it without modulation on the video floppy disc along with the image signal. However, if the whole ID signal to be recorded is changed into an unmodulated continuous carrier signal, no data for the year, month, day, hour, minute, second, etc. can be recorded. Then, in cases where a reproducing operation is to be performed by a reproducing apparatus arranged in conformity to the currently practiced still video format, this solution would cause inconvenience.